The present invention relates to the field of medical monitoring instrumentation in general, and in particular, to the field of labor monitoring devices.
It is well known that the process of labor and childbirth entails the gradual thinning and dilation of the uterine cervix, in response to coordinated uterine contractions, resulting in the eventual expulsion of the fetus through the fully dilated uterine cervix and birth canal. As such, the degree, and rate of progression, of cervical thinning and dilation is closely monitored by hospital staff during labor and childbirth, and is considered to be the cardinal indicator of the progression of labor. Slow or inadequate cervical thinning and dilation may indicate inefficient or pathological labor, and is often an indication for medical or surgical intervention, particularly if accompanied by evidence of fetal distress.
FIG. 1 demonstrates the normal process of cervical dilation and thinning. As can be seen, the cervix thins and dilates as it is "pulled up" into the uterus. Although the normal gravid cervix is slightly soft to the touch, it remains thick, rigid, and closed until a few weeks before the end of gestation, measuring approximately 2-3 cm in length. Several weeks before the onset of labor, the cervix begins to become perceptibly softer. During the process of active labor, the cervix becomes progressively softer and thinner, a process known as effacement. As effacement progresses, the cervix also gradually dilates. Dilation does not ordinarily proceed at a constant rate. Typically, dilation is slow until 4-5 cm dilation is reached, more rapid until dilation is nearly complete, and then slower again until full dilation of 10 centimeters is achieved. Thereafter, the second stage of labor, during which the fetus passes through the dilated cervix and birth canal, begins.
Cervical dilation is recorded in centimeters. Effacement of the cervix is recorded as a percentage: an uneffaced (0%) cervix is firm and about 2.5 cm long; 50% effacement implies that the cervix is about 1 cm thick and somewhat softer; a completely effaced cervix (100%) is soft and only a few millimeters thick. In addition, the consistency of the cervix to touch during manual vaginal examination, that is, whether it is firm or soft, is also an indicator of the process of effacement. Measuring the actual transmural thickness of the wall of the cervix, that is, from the endocervical or uterine surface to the outer vaginal surface, in millimeters, would provide a precise and accurate description of the degree of cervical effacement, however such evaluation is often not feasible by manual vaginal examination.
Due to its cardinal importance as an indicator of the progress of labor, the amount of cervical dilation and effacement is regularly determined by professional attendants (such as midwives and obstetricians) during the course of labor. On average, ten manual vaginal examinations are required per labor. The data is usually plotted on a labor observation chart, as illustrated in FIG. 2. On the illustrated chart, "station" refers to a description of the location of the fetal head in relation to the maternal pelvis. It can be seen that cervical dilation progresses as the fetus descends through the maternal pelvis.
Manual vaginal examinations, however, suffer from several disadvantages: they are embarrassing and uncomfortable for the patient, subjective, inaccurate, provide very intermittent information, can introduce infection into the uterus, require manual charting, and add to the medical staff workload.
Although several monitoring devices are routinely used during the course of labor (such as fetal heart rate monitors, fetal oxygen saturation monitors pulse oxymetry!, uterine activity monitors tocometry!, and maternal vital signs monitors), automatic monitoring of cervical dilation and effacement is not yet available. This is despite the fact that several methods for measuring cervical dilation have been described. These methods include:
1) Obstetric gloves incorporating a measuring string or measuring tape.
2). Finger mounted angular V calipers.
This device is attached to the obstetricians fingers, and is inserted into the vagina whenever a measurement is required. The obstetrician measures dilation by spreading his fingers in the same way as done in regular manual vaginal examination. A mechanical scale, potentiometer, or strain gauge measures the angle between the caliper arms, and the measurement is converted to a dilation value. Both of the above devices suffer from the deficiencies described above for standard manual vaginal examinations.
3) Cervix mounted angular V calipers.
This device is placed in the vagina and attached to two points on opposite sides of the cervix to each other. A mechanical scale, potentiometer, or strain gauge then measures the angle between the caliper arms, and the measurement is converted to a dilation value. This device suffers from the deficiency that is substantially occupies the vagina, thus interfering with other monitoring and treatment activities. It therefore has to be removed and reinserted repeatedly. In addition, it is uncomfortable to the patient, may require manual charting, and is difficult to install, thus adding to the medical staff workload.
4) Induction transmitters and receivers clamped to two sides of the cervix opposite to each other.
As the distance between the primary and secondary induction windings affects the induced signal, the distance between them (which is equal to the cervical diameter) can be measured. This device suffers from the deficiency that it functions effectively only until about 5-7 cm dilation, whereas manual measurements are required until 10 cm dilation. In addition, the insertion of other instruments into the vagina and the cervix, as is often done during labor, adversely effects the measurement readings of the device, rendering it impractical for clinical use.
5) A multi-switch membrane that is inserted into the uterus, and pressed between the cervical internal os and the fetal head.
As the switches in the cervical opening are not pressed, while those within the uterus are, an ongoing indication of the progress of dilation is rendered. This device suffers from several deficiencies. Firstly, it is necessary to insert the membrane between the fetus and the uterine wall, which is technically difficult, particularly before the amniotic sack has ruptured, and undesirable afterwards due to the risk of introducing infection. Secondly, the membrane often shifts, giving rise to false measurements. In addition, as the membrane crosses the cervix from one side to the other after placement, the entrance to the uterus is obstructed, thus interfering with other monitoring and treatment activities. Other disadvantages are that the device is uncomfortable for the patient, and adds to the medical staff workload.
6) Untrasound visualization of the cervix.
Ultrasound monitoring of cervical dilation suffers from the following deficiencies: the probe inserted into the vagina is relatively large (and thus must be removed to allow other monitoring and treatment activities), the ultrasound machine is expensive and complicated to operate, ultrasound radiation that is applied for many hours represents a potential hazard to the fetus, the probe is uncomfortable for the patient, the device is inaccurate, and the device can introduce infection into the uterus.
There is therefore a need for an instrument which is capable of continuously, accurately, and automatically monitoring and recording the progress of cervical dilation and effacement. Such an instrument should be safe, easily installable, and comfortable to the mother. Furthermore, such an instrument should not interfere with other monitoring or treatment procedures commonly performed during labor.